This invention relates to telephone hybrids, which are used to separate signals that flow over a line in both directions into component uni-directional signals.
FIG. 1 depicts the basic prior art arrangement for realizing a transformer-less hybrid circuit as gleaned, for example, from U.S. Pat. No. 4,056,719 issued Nov. 1, 1977 and assigned to the assignee of this invention. FIG. 1 depicts a central office driver 20 that is connected to customer apparatus 10 via impedance 21, which impedance represents the wire resistance from the customer apparatus to the central office. The customer apparatus signal that is destined to the central office is developed by driver 30, which is connected to the central office line (and impedance 21) via resistor 31. The side of resistor 31 that is closer to amplifier 30 is connected to the negative input of amplifier 40 through resistor 32 and the other side of resistor 31 is connected to the positive input of amplifier 40. The output of amplifier 40 is fed back to the negative input of amplifier 40 via resistor 33.
The signal across resistor 31 is a combination of the current induced by driver 30 and by driver 20. It can be shown, however, when R.sub.33 R.sub.32 =R.sub.32 R.sub.21, the output of amplifier 40 is equal to the output of driver 20, thus developing a signal that is devoid of the driver 30 signal. Although the FIG. 1 circuit eliminates the contribution of driver 30 from the output of amplifier 40 (this contribution is often called the "near end echo") it is also known that a portion of the signal of driver 30 that travels to driver 20 reflects off driver 20 because of an impedance mismatch and returns to the customer's apparatus. This portion, which is often called the "far end echo" can not be readily distinguished from the true output of driver 20 and cannot be easily removed from the output of amplifier 40. Far-end echoes can be canceled, however, with an echo canceler that follows amplifier 40 and which trains on the particular characteristic of the signal path from driver 20 to amplifier 40. It appropriately subtracts from the signal arriving from amplifier 40 a filtered portion of the driver 30 output signal.
Echo cancelers are linear circuits, however, and therefore, nonlinearities that are introduced into the far end echo by the transformer cannot be canceled out.
In applications where ground isolation of the central office from the customer equipment is desired, a single isolator can be inserted between resistor 31 and the central office line. As mentioned above, however, any nonlinearities that are introduced by such an isolator cannot be eliminated by the action of resistor 31 and cannot be eliminated by echo cancelers. To overcome this problem, the aforementioned U.S. Pat. No. 4,056,719 discloses a transformer-less arrangement which employs opto-isolators in both paths. In particular, one opto-isolator is placed prior to driver 30 and the other opto-isolator is placed following amplifier 40. That means, of course, that the power supply voltages used to operate driver 30 and amplifier 40 have a common ground with driver 20.
Two problems arise in this arrangement. First, the power supply for driver 30 and amplifier 40 must be developed from the signal arriving at the customer apparatus via impedance 21. Since any dc current drawn for the purpose of powering elements 30 and 40 also causes a voltage drop across impedance 21, there is a limitation on the maximum dc power supply that can usefully be created for elements 30 and 40. In turn, a limitation of the dc supply that powers driver 30 limits the voltage excursions at the output of amplifier 30. The second problem with the FIG. 1 arrangement is that the power transferred to the central office from driver 30 is further limited by the power lost in resistor 31.